I. FIELD OF THE INVENTION
The present invention relates to an automatic speed control system for use with variable speed machine tools such as vertical mills, drill presses, and similar tools which employ quick change tool holders. The system is especially well suited for retrofit coupling to variable speed machine tools of the type which employ mechanically biased, variable ratio belt and pulley drive systems.
II. DESCRIPTION OF THE PRIOR ART
The design of modern vertical milling machines, such as those manufactured by Bridgeport Machine Company, require that the machine operator manually select the spindle drive speed after referencing separate charts or the operator's memory. This speed selection is accomplished by the operator selecting the desired tool from among those available from a tool storage area. The operator then inserts the tool together with its quick change holder into the spindle or chuck which is used to rotate the tool. The actual speed change function is accomplished by having the operator mechanically rotate a crank which turns a worm gear which in turn drives a shaft having a flexible chain coupled thereto. This chain pulls a lever which compresses the opposing effective surfaces of a slave pulley, thereby causing the pulley to assume a larger effective diameter. This slave pulley is coupled by a flexible belt to a drive pulley which is driven by an electromechanical motor. Since the drive pulley includes a mechanically biased pulley wheel of variable diameter, the increase in the effective diameter of the slave pulley causes a corresponding decrease in the effective diameter of the drive pulley, thereby continuously varying the effective drive speed of the spindle. The effective drive speed of the spindle is selected by the operator from a mechanical readout which is actuated responsive to the relative effective diameter of the slave pulley and is mounted upon the head of the machine for convenient optical inspection.
One disadvantage of this system design is that the machine operator is required to consult additional references for the precise operating speed for the tool which has been chosen. This allows some margin for operator error which can damage the machine and lead to unacceptable production tolerances in the items which are produced.
In contrast to the prior art, the present invention envisions an automatic selection of the machine speed responsive to the operator selecting the appropriate tool from the tool storage area. In the tool storage area each tool is located adjacent to a tool presence sensor which is actuated upon the removal of the tool for coupling into the spindle or chuck. This tool sensor automatically selects a predetermined speed signal responsive to the tool which has been selected. This predetermined speed signal is communicated to the speed control circuitry which regulates the effective drive diameter of the slave pulley responsive thereto. The electronic assembly includes a circuit for sensing the relative effective diameter of the slave pulley and responsive thereto corrects any spindle speed variations representing deviations from the selected speed interval. This electronic circuitry also provides means for an electronic speed display for the convenience of the operator.